Numbers Without Repeated Digits

Medium

You get an integer digits. Count the integers x with 0 ≤ x < 10^digits whose decimal form has no repeated digits, meaning each digit appears at most once. Numbers are written the usual way, without leading zeros.

Example 1

digits=1 → 10

The numbers 0 through 9 are single digits with nothing to repeat: 10 of them.

Example 2

digits=2 → 91

Among 0 to 99, only 11, 22, ..., 99 repeat a digit, which leaves 100 − 9 = 91.

Example 3

digits=0 → 1

Constraints

0 ≤ digits ≤ 8

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Topics

Math Dynamic Programming Backtracking

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Input

digits=1

Expected

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